Gravitationally bound ideal gas sphere in Newtonian and in Einsteinian gravity

The gravitational hydrostatic equilibrium of an isothermal ideal gas with pressure proportional to density is investigated. The system is known from the literature as the singular isothermal sphere since it has infinite central density and infinite total mass. In spite of these unphysical features the system is of pedagogical interest since it allows exact mathematical treatment both within Newtonian gravity and within general relativity. The difference between these two theories is nicely illustrated. (paper)